1. Field of the Invention
The present invention relates to a pneumatic tire in which a tread surface is provided with a plurality of main grooves which extend in a tire circumferential direction, and a shoulder block row which is arranged in an outer side in a tire width direction of a shoulder main groove positioned in an outermost side in the tire width direction in the main grooves.
2. Description of the Related Art
There has been known a structure in which an outer side in a tire width direction of a shoulder main groove is provided with a shoulder block row formed by arranging a plurality of blocks in a tire circumferential direction. Generally, an outer diameter is smaller in both end sides of a tread surface than in a center, and the block tends to generate a slip in the shoulder block row due to the diameter difference. Particularly, since a trailing side portion of the block elastically deforms largely in a tire circumferential direction at a time of grounding, a slip amount becomes large. Asa result, the trailing side portion of the block wears by priority, and an irregular wear called as a heel and toe wear is generated.
Further, in the case where a shoulder main groove 10 is inclined such as FIG. 7, the heel and toe wear has been significant in a block 3 which constructs a shoulder block row 4. The inventor of the present invention has made a study of this, and has found that a slip direction of a main groove side portion of the block 3 changes and a slip amount becomes large, from a leading side (a forward side of a tire rotating direction R) thereof toward a trailing side (a rear side of the tire rotating direction R). FIG. 7 shows a direction and a magnitude of the slip by arrows, and a region in which the wear becomes significant is colored by black.
The phenomenon mentioned above becomes significant in the small block 3 which is adjacent to the leading side of the relatively larger block 2. This can be thought to be caused by a fact that a friction energy is lower in the leading side portion of the block 2, the friction energy is higher in the trailing side portion of the block 3, and an energy difference is large, in the shoulder block row 4. The friction energy in this case is a sum of a friction energy in a tire circumferential direction and a friction energy in a tire width direction, and these friction energies come to a product of a stress in each of the directions and a slip amount.
Patent Document 1 and Patent Document 2 describe a pneumatic tire in which a block constructing a shoulder block row is connected to a block positioned in an inner side in a tire width direction by a protrusion portion which is provided in a groove bottom of a shoulder main groove. Further, Patent Document 3 describes a pneumatic tire in which a shoulder rib provided with a lug groove open in a shoulder main groove is provided with a convex portion which reinforces a side wall of a portion partitioned by the lug groove entirely.
However, in the case where the protrusion portion or the convex portion as mentioned above are applied to the block 3 in FIG. 7, a slip direction of the main groove side portion of the block 3 moves to an outer side in a tire width direction (a left side in FIG. 7) while heading for the trailing side. In accordance with this, such as an example mentioned later, the region in which the wear becomes significant is somewhat reduced, however, the trailing side portion of the block 3 wears widely and the heel and toe wear is generated. Further, since the protrusion portion and the convex portion as mentioned above comes to an obstacle for drainage by the shoulder main groove, causing a deterioration of a drainage performance.
Patent Document 4 describes a pneumatic tire in which a reinforcing portion which goes beyond a center line of a main groove is provided in an end portion in a circumferential direction of a block which constructs a shoulder block row. However, even if the reinforcing portion mentioned above is provided in the trailing side of the block 3 in FIG. 7, a slip amount does not change in a leading side and a center in the main groove side portion of the block 3. Accordingly, it is impossible to sufficiently suppress a heel and toe wear.